Technology update

Feb 1, 2007

Tiny mass detectors come out of the cold

Physicists in the US have made the first "nanocantilever" that can measure extremely small masses under everyday conditions – unlike previous such devices that needed a high vacuum or cryogenic temperatures to work. The result, which comes from Michael Roukes and colleagues at the California Institute of Technology, could revolutionize the way tiny particles are weighed for applications in medicine and sensors.

Existing nanoelectromechanical devices are capable of weighing objects with a mass of just 10-18 grams (attograms). However, all such devices require extremely controlled conditions in which to work, such as high vacuum, extremely low temperatures and superconducting magnets. This is because it is difficult to detect the miniscule mechanical motion of the tiny machines, which typically consist of an oscillating cantilever made from a small wafer of semiconducting material a few microns long and several hundred nanometres wide.

The Caltech team's approach is fundamentally different in that it has used metallic films as the sensing layer material in its cantilevers instead of the commonly used semiconductors. The researchers say that their technique overcomes a mindset in sensing that has thwarted the realization of such nanoscale sensors so far. "This change not only greatly simplifies the fabrication process, but also enables us to make extremely small working devices at the nanoscale," explains team member Mo Li. Equally important is the fact that the researchers can sensitively detect the mechanical motion of their devices, even when the machines are moving at very high frequencies of between 30MHz and 300MHz – something that was impossible with cantilevers made from semiconducting materials.

Another unexpected property of these nanocantilevers, compared with microcantilevers, is their substantially reduced viscous damping when they are moving in the atmosphere. This is because of their small cross-sectional dimensions of just 400 nm wide by 80 nm thick, which approaches the mean free path of air molecules in the atmosphere (around 65 nm). The nanodevices can therefore operate under ambient conditions.

Roukes and colleagues demonstrated that their cantilevers can measure masses on the attogram scale with a resolution of just 100 zeptograms (10-19 grams) – a new record under these conditions.

"These nanocantilevers are very versatile platforms for diverse sensing applications," Li told nanotechweb.org. "One ongoing project in our group has already shown that these devices are very sensitive, fast chemical gas sensors that could be used in chemical weapons detection, for example." Other potential applications include electromechanical "noses" for breath analysis and early disease diagnosis, batch fabricated pressure sensors and even components for pace makers, he added.

The Caltech physicists would now like to differentially functionalize hundreds of individual nanodevices within a large array to make the electromechanical equivalent of a dog's nose. For chemical and biological sensing applications, their aim is to tailor each cantilever in the array to detect a specific target species.